1. Field of the Invention
The present invention relates to charged-particle beam systems, including those used for inspection or review of substrates, such as semiconductor wafers and masks.
2. Description of the Background Art
Charged-particle beam systems include electron beam (e-beam) systems, such as scanning electron microscopes (SEMs) and other e-beam instruments. In one application, these systems may be used for inspection and review of substrates, such as semiconductor wafers.
In a typical application of a charged-particle beam system (such as, for example, an SEM), the ability to focus the particle beam into a small spot onto the surface of a specimen is critical to obtaining sharp images or scan profiles of the specimen with a high degree of resolution. The ability to focus thus translates into measurement accuracy and measurement precision.
FIG. 1 is a flow chart of a conventional focusing method 100 for a charged-particle beam system. In the conventional technique, the charged-particle beam is targeted (102) at areas on the specimen that contain topologically varying structures or features (such as, for example, lines, trenches, holes, post or defects). A set of specimen images or scan profiles are acquired (104) from these targeted areas by sweeping through the current settings of the objective lens of the system or the bias voltages applied to the specimen. Changing either the objective lens current or specimen bias voltage through several steps effectively changes the focal point of the beam to a different height at each step. A sharpness measure of the image or profile at each lens current or bias voltage setting is then computed (106) to generate (108) a sharpness curve. Typically, the sharpness measure may be computed (106) using a square (or magnitude) of a first (or second) derivative of the image or profile signal. A best focus condition is determined (110) from the maximum of the sharpness curve. In other words, the lens current or bias voltage value at that maximum is the setting at which the image or scan profile is best focused.
The computation of the sharpness measure typically involves filtering (105) the image or scan profile through a band-pass filter. The pass band of the filter is set such that only the signal energy associated with a selected spatial frequency range is extracted and used to derive the sharpness measure. The pass band typically selects a middle range of spatial frequencies that efficiently represent the edge information embodied in the physical features and therefore is most sensitive to a change in focus. The data from the lower spatial frequency range represents relatively flat or slowly varying features that are relatively insensitive to a change in focus. The data from the higher spatial frequency range may be dominated by noise. These lower frequency data and higher frequency data are conventionally discarded.
Unfortunately, the above-discussed conventional focusing technique has drawbacks and limitations. These drawbacks and limitations are discussed below.
It is desirable to improve charged-particle beam systems, including those utilized for the automated inspection or review of substrate surfaces. More particularly, it is desirable to improve focusing capabilities of charged-particle beam systems.